Fuel System Diagnosis System Check
Circuit Description
When you turn ON the ignition switch, the Vehicle Control Module (VCM) turns ON the in-tank fuel pump. The pump remains ON as long as the engine is cranking or running and the VCM is receiving reference pulses. If there are no reference pulses, the VCM shuts the fuel pump OFF within 2 seconds after the ignition was switched to the ON position or if the engine stops. The VCM will also turn ON the fuel pump for 2 seconds when the ignition is turned to the OFF position.
An electric fuel pump pumps the fuel through an in-line filter to the Central SFI unit. The pump is attached to the fuel level meter assembly inside of the fuel tank. The pump is designed to provide fuel pressure above what is needed by the fuel injectors. The pressure regulator keeps the fuel available to the injectors at a regulated pressure. Unused fuel is returned to the fuel tank by the fuel return pipe.
Diagnostic Aids
Tools Required
| • | J 34730-1A Fuel Pressure Gage in J 34730-E or |
| • | J 34730-1A Fuel Pressure Gage with J 34730-250 Fuel Pressure Adapter Kit. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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Wrap a shop towel around the fuel pressure connection to absorb any small amount of fuel leakage that may occur when installing the fuel pressure gauge. Turn ON the ignition, the fuel pressure should be 415-455 kPa (60-66 psi) for the Central SFI and 385-430 kPa (56-62 psi) for the SFI. This pressure is controlled by a spring pressure within the regulator assembly.
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The fuel pressure that continues to fall is caused by one of the following items:
| • | The in-tank fuel pump check valve not holding. |
| • | A partially disconnected fuel pulse dampener (pulsator). |
| • | The fuel pressure regulator valve leaking. |
| • | A central SFI injector and poppet valve leaking. |
| • | An SFI injector is leaking |
-
When the engine is idling, the manifold pressure is low (high vacuum) and is applied to the fuel pressure regulator diaphragm. This will offset the spring and result in a lower fuel pressure. This idle pressure will very somewhat depending on the barometric pressure; however, the pressure idling should be less indicating the pressure regulator control.
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The fuel pressure less than 415 kPa (60 psi) for Central SFI or 385 kPa (56 psi) for SFI, falls into the following 3 areas:
| • | A regulated pressure but less than 415 kPa (60 psi) for Central SFI or 385 kPa (56 psi) for SFI. Amount of fuel to injector OK, but pressure is too low. System will run lean and may set a DTC. Also, hard starting cold and overall poor performance or may not start at all. Refer to Engine Cranks but Does Not Run |
| • | A restricted flow causing pressure drop -- Normally, a vehicle with a fuel pressure of less than 300 kPa (44 psi) at idle is undrivable. However, if the pressure drop occurs only while driving, the engine will normally surge then stop running as the pressure begins to drop rapidly. This is most likely caused by a restricted fuel line or plugged filter. |
Step | Action | Value(s) | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 |
Important: Before clearing DTCs, use the scan tool in order to record the freeze frame and the failure records for reference because the Clear Info function will lose the data. Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||||||||||
Important: The ignition may have to be cycled on more than once in order to obtain the maximum fuel pressure. Is the fuel pressure within the specified value? | Central SFI - 415-455 kPa (60-66 psi) SFI - 385-430 kPa (56-62 psi) | |||||||||||
3 |
Does the fuel pressure hold steady within the specified value? | Central SFI - 415-455 kPa (60-66 psi) SFI - 385-430 kPa (56-62 psi) | ||||||||||
Does the fuel pressure hold steady within the specified value? | Central SFI - 415-455 kPa (60-66 psi) SFI - 385-430 kPa (56-62 psi) | |||||||||||
Is the fuel pressure at the specified value? | Central SFI - 415-455 kPa (60-66 psi) SFI - 385-430 kPa (56-62 psi) | Go to Driveability Symptoms | ||||||||||
6 | Check for a partially disconnected fuel pulse dampener (Pulsator). Was a problem found? | -- | ||||||||||
7 | Repair connection at the fuel pulse dampener (pulsator). Refer to Wiring Repairs in Engine Electrical. Is the action complete? | -- | -- | |||||||||
8 | Inspect the fuel feed line between the pinch and the throttle body. Was a problem found? | -- | ||||||||||
9 |
Is fuel pressure present? | -- | ||||||||||
10 |
Listen for the fuel pump running. Does the fuel pump run? | -- | ||||||||||
11 | Perform the following checks:
Was a problem found? | -- | ||||||||||
12 |
Is the action complete? | -- | ||||||||||
13 | Replace the in-tank fuel pump. Refer to Fuel Pump Replacement (Main) . Is the action complete? | -- | -- | |||||||||
14 |
Is the fuel pressure above the specified value? | Central SFI - 455 kPa (66 psi) SFI - 430 kPa (62 psi) | ||||||||||
15 |
Is the fuel pressure above the specified value? | Central SFI - 415 kPa (60 psi) SFI - 385 kPa (56 psi) | ||||||||||
Is the fuel pressure below the specified value | Central SFI - 415 kPa (60 psi) SFI - 385 kPa (56 psi) | |||||||||||
17 | Check for a restricted fuel line from the fuel pressure regulator to the point where the fuel line was disconnected. Is the fuel line restricted? | -- | ||||||||||
18 | Repair the restriction in the fuel line from the fuel pressure regulator to the point where the fuel line was disconnected. Is the action complete? | -- | -- | |||||||||
19 | Repair the restriction in the fuel return line to the fuel tank. Is the action complete? | -- | -- | |||||||||
20 | Perform the following items:
Was a problem found? | -- | ||||||||||
21 | Repair as necessary or replace as necessary the following items:
Is the action complete? | -- | -- | |||||||||
Notice: DO NOT allow the fuel pressure to exceed 517 kPa (75 psi). Excessive pressure may damage the fuel system. Is the fuel pressure above the specified value? | Central SFI - 455 kPa (66 psi) SFI - 385 kPa (56 psi) | |||||||||||
23 | Replace the fuel pressure regulator. Refer to Fuel Pressure Regulator Replacement (Central SFI) for the 5.0L, 5.7L. Refer to Fuel Pressure Regulator Replacement for the 7.4L. Is the action complete? | -- | -- | |||||||||
24 | Is the fuel pressure less then the specified value? | Central SFI - 415 kPa (60 psi) SFI - 385 kPa (56 psi) | ||||||||||
25 | Perform the following items:
Was a problem found? | -- | ||||||||||
26 | Repair or replace as necessary:
Is the action complete? | -- | -- | |||||||||
27 | Replace the fuel feed line. Is the action complete? | -- | -- | |||||||||
28 |
Does the engine start and continue to run? | -- | ||||||||||
29 |
Are any DTCs displayed? | -- | Go to The Applicable DTC Table | |||||||||
30 | Using the scan tool, select the Capture Info and the Review Info. Are any DTCs displayed that have not been diagnosed? | -- | Go to The Applicable DTC Table | System OK |
Fuel System Diagnosis Fuel Supply System
Alcohol-In-Fuel
Notice: Do not use fuels containing methanol in order to prevent damage and corrosion to the fuel system.
Alcohol-in-fuel can be detrimental to the fuel system components. Alcohol-in-fuel can cause driveability problems such as hesitation, lack of power, stall, no start, etc.
The problems may be due to fuel system corrosion and subsequent fuel filter plugging, deterioration of rubber components, and air fuel mixture leaning.
Various types and concentrations of alcohol are used in commercial fuel. Some alcohol is more detrimental to fuel system components than others. If an excessive amount of alcohol in the fuel is suspected as the cause of a driveability condition, the following procedure may be used in order to detect the presence of alcohol in the fuel. In this procedure, water is used to extract the alcohol from the fuel. If contaminated fuel is suspected, the fuel must be completely drained from the tank and replaced with known good fuel.
Testing Procedure
The fuel sample should be drawn from the bottom of the tank so that any water present in the tank will be detected. The sample should be bright and clear. If the sample appears cloudy, or contaminated with water (as indicated by a water layer at the bottom of the sample), this procedure should not be used and the fuel system should be cleaned. Refer to Fuel System Cleaning (Purge the Fuel Tank) and refer to Fuel System Cleaning (Purge Feed and Return Pipes) for V6 applications. Refer to Fuel System Cleaning (Purge the Fuel Tank) and refer to Fuel System Cleaning (Purge Feed and Return Pipes) for V8 applications.
- Using a 100 ml cylinder with 1 ml graduation marks, fill with fuel to the 90 ml mark.
- Add 10 ml of water to bring the total fluid volume to 100 ml.
- Install a stopper.
- Shake vigorously for 10 to 15 seconds.
- Carefully loosen the stopper to release the pressure.
- Close the stopper.
- Shake vigorously again for 10 to 15 seconds.
- Put the graduated cylinder on a level surface for approximately 5 minutes in order to allow adequate liquid separation.
If alcohol is present in the fuel, the volume of the lower layer (which would now contain both alcohol and water) will measure greater than 10 ml.
For example, if the volume of the lower layer is increased to 15 ml, it will indicate at least 5% alcohol in fuel. The actual amount of alcohol may be somewhat greater because this procedure does not extract all of the alcohol from the fuel.
Fuel Pump
Refer to Fuel System Diagnosis .
An inoperative fuel pump would cause a no start condition. A fuel pump which does not provide enough pressure can result in poor performance. Refer to Fuel System Diagnosis .
Fuel Filter
A plugged fuel filter may cause a restricted fuel delivery or a no start condition.
Fuel Pipes and Hoses
The diagnosis of gasoline odor may be a condition of a leaking fuel feed, a return pipe or hose. Fuel pipes that are pinched, plugged, or mis-routed may cause restricted fuel delivery.
Fuel Tank(s)
The diagnosis of gasoline odor may be a condition of leaking fuel tank, filler neck, or filler cap.
A defective filler cap, a plugged or pinched vapor pipe can cause a collapsed fuel tank.
Loose mounting straps, or foreign material in tank, may cause a rattle at the fuel tank.
Fuel Tank Leak Test
Important:
• Before attempting Fuel Tank Leak Check place a dry chemical (Class
B) fire extinguisher near work area. • Before removing the fuel tank for a suspected leak, make sure
the fuel pipes or tubes are not leaking onto the tank. Once removed, make
sure fuel is not leaking around the fuel sender assembly O-ring. • This check requires the fuel sender assembly with O-ring to be
installed.
- Disconnect the battery cable.
- Relieve the fuel system pressure. Refer to Fuel Pressure Relief Procedure .
- Drain the fuel tank. Refer to Fuel Tank Draining Procedure .
- Remove the fuel tank.
- Cap the fuel feed tube, fuel return tube and the vapor tube on the fuel sender assembly.
- Connect a piece of hose to the filler tube nipple and plug the opposite end (if equipped).
- Submerge the tank in water or apply a soap solution to the outside of the tank.
- Apply 35 kPa (5 psi) of air pressure to the vent hose on the fuel tank.
- Air bubbles appearing from the fuel tank indicate a leak. Replace the fuel tank if it is leaking.
| • | Refer to Fuel Tank Replacement (Suburban) for the Suburban. |
| • | Refer to Fuel Tank Replacement (Tahoe/Yukon) for the Tahoe/Yukon. |
| • | Refer to Fuel Tank Replacement (Extended Cab - Side Tank) for the Extended Cab. |
| • | Refer to Fuel Tank Replacement (Pick-up - Side Tank) for the Pick-up. |
| • | Refer to Fuel Tank Replacement (Cab and Chassis - Side Tank) for the Cab and Chassis with a side tank. |
| • | Refer to Fuel Tank Replacement (Cab and Chassis - Rear Tank) for the Cab and Chassis with a rear tank only. |
| • | Refer to Fuel Tank Replacement for the 4DR Utility. |
| • | Refer to Fuel Tank Replacement (Suburban) for the Suburban. |
| • | Refer to Fuel Tank Replacement (Tahoe/Yukon) for the Tahoe/Yukon. |
| • | Refer to Fuel Tank Replacement (Extended Cab - Side Tank) for the Extended Cab. |
| • | Refer to Fuel Tank Replacement (Pick-up - Side Tank) for the Pick-up. |
| • | Refer to Fuel Tank Replacement (Cab and Chassis - Side Tank) for the Cab and Chassis with a side tank. |
| • | Refer to Fuel Tank Replacement (Cab and Chassis - Rear Tank) for the Cab and Chassis with a rear tank only. |
| • | Refer to Fuel Tank Replacement for the 4DR Utility. |
A malfunction of the fuel balance control system may result in a Cranks But Will Not Run condition, caused by an inaccurate fuel gauge. Diagnosis of the fuel gauge can be found in Electrical Diagnosis.
Accelerator Control
Check for correct cable routing or binding. Correct as necessary.
Fuel System Diagnosis Fuel Metering Systems
Fuel Pump Relay
The fuel pump relay is mounted in the underhood electrical center located in the engine compartment. For diagnosis of the fuel pump relay circuit, refer to Fuel Pump Circuit Diagnosis .
Fuel Pump Engine Oil Pressure Switch
The fuel pump and engine oil pressure indicator switch is mounted in the engine block near the distributor. For diagnosis of the fuel pump circuit, refer to Fuel Pump Circuit Diagnosis .
Fuel Metering System
Some failures of this system will result in an Engine Cranks But Will Not Run symptom. If this condition exists, refer to Engine Cranks but Does Not Run .
This table determines if the problem is caused by the ignition system, VCM, or fuel pump circuit. If it is determined to be a fuel problem, refer to Fuel System Diagnosis .
This includes the fuel injector poppet assembly, fuel pressure regulator, fuel pump and fuel pump relay. The fuel system wiring schematic diagram is covered in Fuel Pump Circuit Diagnosis. Refer to Fuel Pump Circuit Diagnosis .
If a problem occurs in the fuel metering system, it usually results in either a rich or lean exhaust condition. This condition is sensed by the HO2S. This condition causes the VCM to change the fuel calculation (injector pulse width). The change made to the fuel calculation is indicated by a change in the short and long term fuel trim values which can be monitored by a scan tool. A momentary change to the fuel calculation is indicated by the short term fuel trim value, while a prolonged change is indicated by the long term fuel trim value. Average fuel trim values will measure around 128. The averages may vary slightly from engine to engine.
Important: When using a scan tool to observe fuel trim values, remember that if the system is in control, no action is required unless a driveability symptom is present.
Listed below are examples of lean and rich HO2S signals with the system in control and out of control.
| • | A momentary lean HO2S signal (system is in control) will appear on the scan tool as the following items: |
| - | Short term fuel trim value above 128 (adding fuel). |
| - | Long term fuel trim value around 128. |
| • | A prolonged lean HO2S signal (system is in control) will appear on the scan tool as the following items: |
| - | Short term fuel trim value around 128. |
| - | Long term fuel trim value above 128 (added fuel). |
| • | A prolonged lean HO2S signal (system is out of control) will appear on the scan tool as the following items: |
| - | Short term fuel trim value well above 128 (adding fuel). |
| - | Long term fuel trim value well above 128 (added fuel). |
If both fuel trim values are fixed well above 128, see DTC P0131 for items which can cause a lean system. Refer to DTC P0133 HO2S Slow Response Bank 1 Sensor 1 .
| • | A momentary rich HO2S signal (system is in control) will appear on the scan tool as the following items: |
| - | Short term fuel trim value less than 128 (reducing fuel). |
| - | Long term fuel trim value around 128. |
| • | A prolonged rich HO2S signal (system is in control) will appear on the scan tool as the following items: |
| - | Short term fuel trim value around 128. |
| - | Long term fuel trim value less than 128 (reduced fuel). |
| • | A prolonged rich HO2S signal (system is out of control) will appear on the scan tool as the following items: |
| - | Short term fuel trim value much less than 128 (reducing fuel). |
| - | Long term fuel trim value much less than 128 (reduced fuel). |
If the fuel trim values are fixed well below 128, see DTC P0132 for items which can cause the system to run rich. Refer to DTC P0132 HO2S Circuit High Voltage Bank 1 Sensor 1 .
If a driveability symptom exists, refer to the particular symptom in Symptoms, for additional items to check.
Fuel System Diagnosis Fuel Control
Fuel delivery is controlled by the control module system.
The diagnosis of fuel control starts with Engine Cranks But Will Not Run. This table will test the fuel system to determine if there is a problem. Refer to Engine Cranks but Does Not Run .
Fuel Injector
Testing of the fuel injector circuit is located in the Engine Cranks but Does Not Run with additional diagnosis in the Injector Circuit Diagnosis .
A fuel injector which does not open may cause a no-start condition. An injector which is stuck partially open could cause loss of pressure after sitting, resulting in extended crank times on some engines. Also, dieseling could occur because some fuel could be delivered to the engine after the key is turned OFF.
Pressure Regulator
Testing the pressure regulator circuit is in the Engine Cranks but Does Not Run and the Fuel System Diagnosis .
If the pressure regulator supplies pressure which is too low, poor performance could result. If the pressure is too high, unpleasant exhaust odor may result.
Idle Air Control (IAC)
The diagnosis of Idle Air Control (IAC) can be found in VCM Outputs Diagnosis .
If the IAC valve is disconnected or connected when the engine is running, the idle RPM may be wrong. The IAC valve may be reset by turning the ignition switch ON for 10 seconds, OFF for 5 seconds.
The IAC valve affects the idle characteristics of the engine as well as throttle follow-up to compensation for sudden throttle closing. If it is open fully too much air will be allowed in the manifold and idle speed will be high. If it is stuck closed, too little air will be allowed in the manifold, and idle speed will be too low. If it is stuck part way open, the idle may be rough, and will not respond to engine load changes.
Fuel Pump Circuit
The relay has a terminal to test the fuel pump operation which is a separate terminal located near the fusible link cluster. By applying voltage at this terminal, it can be determined if the fuel pump will operate. This terminal will also prime the fuel line to the fuel injection unit.
Refer to Fuel Pump Circuit Diagnosis for diagnosis of the fuel pump relay circuit.
An inoperative fuel pump will cause a no start condition. A fuel pump which does not provide enough pressure can result in poor performance.
An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The fuel pump oil pressure switch will turn ON the fuel pump as soon as oil pressure reaches about 28 kPa (4 psi).
Engine Coolant Temperature (ECT) Sensor
The engine coolant temperature sensor is a thermistor (a resistor which changes value based on temperature) mounted in the engine coolant stream. Low coolant temperature produces a high resistance (100,000 ohms at -40°C/-40°F) while high temperature causes low resistance (70 ohms at 130°C/266°F).
The VCM supplies a 5V signal to the engine coolant temperature sensor through a resistor in the VCM and measures the voltage. The voltage will be high when the engine is cold, and low when the engine is hot. By measuring the voltage, the VCM calculates the engine coolant temperature. Engine coolant temperature affects most systems the VCM controls.
The scan tool displays engine coolant temperature in degrees. After engine start-up, the temperature should rise steadily to about 90°C (194°F) then stabilize when thermostat opens. If the engine has not been run for several hours (overnight), the engine coolant temperature and intake air temperature displays should be close to each other. A fault in the engine coolant sensor circuit should set DTC P0117 or DTC P0118. The DTC tables also contain a table to check for sensor resistance values relative to temperature. Refer to DTC P0117 Engine Coolant Temperature (ECT) Sensor Circuit Low Voltage or DTC P0118 Engine Coolant Temperature (ECT) Sensor Circuit High Voltage .
Intake Air Temperature (IAT) Sensor
The Intake Air Temperature (IAT) sensor is a thermistor which changes value based on the temperature of air entering the engine. Low temperature produces a high resistance (100,000 ohms at -40°C/-40°F), while high temperature causes low resistance (70 ohms at 130°C/266°F). The VCM supplies a 5V signal to the sensor through a resistor in the VCM and measures the voltage. The voltage will be high when the incoming air is cold, and low when the air is hot. By measuring the voltage, the VCM calculates the incoming air temperature.
The IAT sensor signal is used to adjust spark timing according to incoming air density.
The scan tool displays temperature of the air entering the engine, which should read close to ambient air temperature when engine is cold, and rise as underhood temperature increases. If the engine has not been run for several hours (overnight) the IAT sensor temperature and engine coolant temperature should read close to each other. A failure in the IAT sensor circuit should set DTC P0112 or DTC P0113. Refer to DTC P0112 Intake Air Temperature (IAT) Sensor Circuit Low Voltage or DTC P0113 Intake Air Temperature (IAT) Sensor Circuit High Voltage .
Manifold Absolute Pressure (MAP) Sensor
DTC P0108 or DTC P0107 indicates a failure in the MAP sensor circuit. Refer to DTC P0107 Manifold Absolute Pressure (MAP) Sensor Circuit Low Voltage or DTC P0108 Manifold Absolute Pressure (MAP) Sensor Circuit High Voltage .
Heated Oxygen Sensor (HO2S 1)
The exhaust Heated Oxygen Sensor (HO2S 1) is mounted in the exhaust manifold where it can monitor the oxygen content of the exhaust gas stream.
The oxygen content in the exhaust reacts with the sensor to produce voltage output. This voltage should constantly fluctuate from approximately 100 mV (high oxygen content - lean mixture) to 900 mV (low oxygen content - rich mixture). The heated oxygen sensor voltage can be monitored with a Scan tool.
By monitoring the voltage output of the heated oxygen sensor, the VCM calculates what fuel mixture command to give to the injector (lean mixture-low HO2S 1 voltage=rich command, rich mixture-high HO2S 1 voltage=lean command).
The heated oxygen sensor circuit, if open, should set a DTC P0134 and the Scan tool will display a constant voltage between 350-550 mV. A constant voltage below 250 mV in the sensor circuit should set DTC P0131, while a constant voltage above 750 mV in the circuit should set DTC P0132. DTC P0131 and DTC P0132 could also be set as a result of fuel system problems. Refer to DTC P0131 HO2S Circuit Low Voltage Bank 1 Sensor 1 or DTC P0132 HO2S Circuit High Voltage Bank 1 Sensor 1 or DTC P0134 HO2S Circuit Insufficient Activity Bank 1 Sensor 1 .
HO2S 2 (Catalyst Monitor)
In order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides of Nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The VCM has the capability to monitor this process using HO2S 2. HO2S 2, located in the exhaust stream past the three-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust emissions effectively. A problem with the HO2S 2 electrical circuits should set DTC P0137, P0138 or P0140, depending on the specific condition. If the catalyst is functioning correctly, the HO2S 2 signal will be far less active than that produced by HO2S 1. If a problem exists which causes the VCM to detect excessive HO2S 2 activity outside of an acceptable range for an extended period of time, the VCM will set DTC P0420, indicating that the three-way catalytic converter's oxygen storage capacity is below a threshold considered acceptable. Refer to DTC P0137 HO2S Circuit Low Voltage Bank 1 Sensor 2 or DTC P0138 HO2S Circuit High Voltage Bank 1 Sensor 2 or DTC P0140 HO2S Circuit Insufficient Activity Bank 1 Sensor 2 or DTC P0420 Catalyst System Low Efficiency Bank 1 .
Throttle Position Sensor
The Throttle Position (TP) sensor is a potentiometer connected to the throttle shaft on the throttle body. By monitoring the voltage on the signal line, the VCM calculates throttle position. As the throttle valve angle is changed (accelerator pedal moved), the TP sensor signal also changes.
At a closed throttle position, the output of the TP sensor is low. As the throttle valve opens, the output increases so that at Wide Open Throttle (WOT), the output voltage should be above 4V.
The VCM calculates fuel delivery based on throttle valve angle (driver demand). A broken or loose TP sensor may cause intermittent bursts of fuel from an injector and unstable idle because the VCM thinks the throttle is moving. A problem in the TP sensor 5V reference or signal circuits should set either a DTC P0122 or DTC P0123. A problem with the TP sensor ground circuit may set DTCs P0123 and P0117. Once a DTC is set, the VCM will use an artificial default value based on mass air flow for TP sensor and some vehicle performance will return. A high idle may result when either DTC P0122 or DTC P0123 is set. Refer to DTC P0122 Throttle Position (TP) Sensor Circuit Low Voltage or DTC P0123 Throttle Position (TP) Sensor Circuit High Voltage or DTC P0117 Engine Coolant Temperature (ECT) Sensor Circuit Low Voltage
TP Sensor Output
This check should be performed when TP sensor attaching parts have been replaced. A scan tool can be used to read the TP signal output voltage.
- Connect the DMM from TP sensor connector terminal B (BLK wire) to terminal C (DK Blue wire). Jumpers for terminal access can be made using terminals 1214836 and 12014837.
- With ignition ON, engine OFF, the TP signal voltage should be less than 1.25 volts if more than 1.25 volts verify free throttle movement. If still more than 1.25V, replace TP sensor.
- Remove the voltmeter and jumpers, reconnect the TP sensor connector to the sensor.
Vehicle Speed Sensor (VSS)
The vehicle speed sensor circuit diagnosis is in DTC P0500 table. Refer to DTC P0500 Vehicle Speed Sensor (VSS) Circuit .
Scan reading should closely match speedometer readings with the drive wheels turning.
Exhaust Gas Recirculation (EGR) System
DTC P0401 or P1406 indicates that there is a failure in the EGR system circuit. Refer to DTC P0401 Exhaust Gas Recirculation (EGR) Flow Insufficient or DTC P1406 Exhaust Gas Recirculation (EGR) Position Sensor Performance .
Idle Speed
When there is a problem with idle air control system or refer to VCM Outputs Diagnosis .
| • | System too lean (High air/fuel ratio) - Idle speed may be too high or too low. Engine speed may vary up and down, disconnecting IAC does not help. May set DTC P0171. Refer to DTC P0171 Fuel Trim System Lean Bank 1 . |
| • | Scan tool and/or voltmeter will indicate a heated oxygen sensor output less than 300 mV (0.3V). Check for low regulated fuel pressure or water in fuel. A lean exhaust with a Heated Oxygen Sensor (HO2S) output fixed above 800 mV (0.8V) may be the result of a contaminated sensor, usually silicone. This may also set a DTC P0172. Refer to DTC P0172 Fuel Trim System Rich Bank 1 . |
| • | System too rich (Low air/fuel ratio) - Idle speed too low. Scan counts usually above 80. System obviously rich and may exhibit black exhaust smoke. Scan tool and/or voltmeter will read a Heated Oxygen Sensor (HO2S) signal fixed above 800 mV (.8V). |
4X Crankshaft Position Sensor
The scan tool scan tool displays crankshaft position sensor data as engine speed (RPM). An error in the crankshaft position sensor circuit should set a DTC P0336, P0337, P0338, or a P0339. Refer to DTC P0336 Crankshaft Position (CKP) Sensor Performance or DTC P0337 Crankshaft Position (CKP) Sensor Circuit Low Duty Cycle or DTC P0338 Crankshaft Position (CKP) Sensor Circuit High Duty Cycle or DTC P0339 Crankshaft Position (CKP) Sensor Circuit Intermittent .
Camshaft Position Sensor
The scan tool scan tool will display camshaft position sensor data as a 0 and 1 as the sensor pulses, the scan data will switch from 0 to 1. All camshaft position sensor data should be checked at idle. An error in the camshaft position sensor circuit should set a DTC P0340. Refer to DTC P0340 Camshaft Position (CMP) Sensor Circuit .
The camshaft position sensor sends a signal to the VCM which uses it as a sync pulse to trigger the injectors in proper sequence.
Cam Signal
The VCM uses this signal to determine the position of the #1 piston during its power stroke. This signal is used by the VCM to calculate fuel injection mode of operation. A loss of this signal will set DTC P0340.
If the cam signal is lost while the engine is running, the fuel injection system will shift to a calculated fuel injection mode based on the last fuel injection pulse, and the engine will continue to run. The engine can be restarted and will run in the calculated mode as long as the fault is present .
Refer to DTC P0340 Camshaft Position (CMP) Sensor Circuit for further information.
4X Reference High Signal
The ignition control module receives signals from the 4X crankshaft position sensor. This signal is sent to the VCM as a 5V digital signal. The VCM uses this input as part of it's final calculation for Ignition Control (IC) timing. Refer to Enhanced Ignition System for more information on the Electronic Ignition (EI) system.
Crankshaft Position Sensor
The crankshaft position sensor provides a signal through the ignition control module which the VCM uses as reference to calculate RPM and crankshaft position. Refer to Enhanced Ignition System for V6 or refer to Enhanced Ignition System for V8 for additional information.
Knock Sensor (KS) System
DTC P0327 will set when KS line is shorted to ground. Refer to DTC P0327 Knock Sensor (KS) Circuit .
Distributor Reference Signal
The distributor reference signal is covered in the ignition system. Refer to Enhanced Ignition System for V6 or refer to Enhanced Ignition System for V8 for additional information..
A/C Clutch Control
Refer to Air Conditioning (A/C) Compressor Clutch Control Diagnosis .
Exhaust System
Refer to Restricted Exhaust System Check .
